OSD design guidelines and checklist

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OSD design system elements

a runoff collection system consisting of gutters, downpipes, pits and pipes

a runoff storage system

an outlet structure to control the rate of stormwater discharge from the runoff storage system to the Council drainage network

Each element is discussed further below.

Runoff collection system

The runoff collection system must collect runoff from all pervious and impervious areas and direct it to the runoff storage system.

Surface inlet pits must be fitted with a suitable grate to prevent litter entering the underground drainage system and causing blockages.

Site drainage must be designed in accordance with Australian Standard AS3500.3:2003 National Plumbing and Drainage Code – Stormwater Drainage.

The Relevant Building Surveyor for the development is responsible for approving the design of the site drainage system in accordance with Regulation 610 of the Building Regulations 2006.

Runoff storage system

The runoff storage system is required to temporarily store stormwater runoff during a storm. The runoff storage system must:

provide the storage volume stated in the planning permit

drain within 72 hours to ensure the storage volume is available for a subsequent storm event The runoff storage system may consist of the following structures (or combination of structures):

below ground tank(s) or pipe(s)

above ground storage in a grassed or landscaped area where the storage volume can be provided with minimal adjustment to ground levels

above ground tank(s) connected to the internal reuse system of a building on the development site

above ground tank(s) fitted with a controlled discharge orifice to maintain an active storage volume within the tank

Each storage system is discussed further below.

Below ground storage system

Below ground storage systems must:

be structurally designed to withstand all service loads and provide a service life of at least 50 years

be located outside the tree protection zone (TPZ) of any trees to be retained

have a minimum soil cover of 300mm in landscaped areas

be fitted with a child proof locking system for the surface grate where the depth of the below ground storage facility exceeds 1,200mm

Above ground storage system

Above ground storage systems must:

not be unsightly or hazardous

be totally impermeable except for infiltration into the ground where soil conditions allow and the system is located at least five metres from any building foundations

have a ponding depth no greater than 200mm in areas where vehicles will be parked or otherwise no greater than 300mm

Rainwater tanks

A rainwater storage tank can be credited toward the total volume of the runoff storage system when the tank is:

connected to the dwelling for regular uses such as toilet flushing and clothes washing, or

fitted with a controlled discharge orifice to maintain an active storage volume within the tank

One-third of the total volume of the tank can count toward overall site storage when the tank is connected to the dwelling for reuse.

The active storage volume of the tank can also count toward the overall site storage when the tank is fitted with a controlled discharge orifice.

Council may reduce the storage volume credited to the rainwater tank in proportion to the amount of roof area that drains to the tank.

Flow control outlet

The flow control outlet must be located within the boundaries of the lot between the storage system and the Council drainage system. The flow control outlet must not be located within a Council drainage easement.

The outlet must limit the rate of flow to the Council drainage system to the permissible site discharge (PSD) when the OSD storage is full (maximum head).

The outlet must be resistant to blockage and must include a trash grate located upstream of the outlet or sufficient screening on all inlets.

The flow control outlet must use either the SVC MC2 Multicell (or approved equivalent) or an orifice and baffle wall arrangement.

The orifice sizing calculation is detailed below.

Outlet control orifice sizing

The sizing of the outlet control orifice is based on the following parameters:

Overland flow

The runoff storage system and flow control outlet structure must be designed to safely convey overflows to an adequate Council overland flow path or drainage system. This requirement may be waived if Council deems that no suitable overland flow path or drainage system is available and the OSD system has been designed to detain the 100 year ARI storm.

OSD design checklist

1. OSD system

Ref

Requirement

Comments

Pass

Fail

NA

1.1

OSD storage volume equals or exceeds required storage volume

Storage =m3

1.2

Outlet orifice controls rate of discharge to permissible site discharge (PSD) when OSD storage is full (maximum system head)

Outlet Q =litres/sec

1.3

Top level of orifice pit baffle wall is set at or above the maximum (design) water level needed to achieve the required storage volume

Maximum water level =m AHD Top of baffle wall level =m AHD

1.4

Storage volume is apportioned between tanks and below ground system according to relative catchment sizes

1.5

Below ground storage system pipes are not located beneath a habitable building

1.6

Orifice pit outlet invert level is above the invert level of the property connection to the legal point of discharge (LPD)